Declaration emit should not inline type definitions

TypeScript Version: 3.8.3, 3.8.1, probably others

Search Terms:

declaration inlining, dts inlining, declaration inline, inline literal, declaration literal

Code

// parent.ts
import { num, obj } from "./child"
export const reExportNum = num;
export const reExportObj = obj;

// child.d.ts
export declare const num: number;
export declare const obj: { a: 1 };

tsc index.ts --declaration

Expected behavior:

Declaration emit for parent.ts should not inline types.

// parent.d.ts
import { num, obj } from "./child"
export declare const reExportNum: typeof num;
export declare const reExportObj: typeof obj;

Actual behavior:

Today, declaration emit for parent.ts inlines the types and eliminates the import of the child.d.ts type definition.

// parent.d.ts
export declare const reExportNum: number;
export declare const reExportObj: {
    a: 1;
};

This is a correctness issue, because consumers of parent.d.ts will not get the correct types if the types in child.d.ts change.

In practice, this is most likely to happen when parent and child are in separate packages, because they are published independently, i.e. an application uses parent-package which uses types from child-package. This is exacerbated by the current practice on npm of parent-package depending on an unpinned version, using package.json dependency syntax "child-package": "*".

This issue was co-authored with @robpalme

[cyberixae]

I’m here because inlining everything caused my .d.ts file to be over 6.5MB long. The type checks started failing and I am wondering if the compiler simply ignores the end of the file because of some limitation.

[robpalme]

@aleksey-ilin the main solution I have found to solve huge declarations is to identify the root type that is inlined and then, assuming it is a statically known object type, create an interface from it and then refer to that interface at all usage sites.

export interface WrappedProblemType extends ProblemType {}

I have been experimenting with changing declaration emit so that shenanigans like this are not necessary. It kinda works and I’ll share that soon.

---

Separately, union and intersection types also get inlined. interface will not save you in this case - there is no reliable userland workaround for these. Thankfully there is work in progress to reduce the inlining of these specific types in https://github.com/microsoft/TypeScript/pull/42149